Respect default SASS load paths

This allows gems providing SASS assets to add their own asset path to the default SASS load_path:

    Sass::Engine::DEFAULT_OPTIONS[:load_paths] << path

Since SASS also looks for imports in the same directory as the importing file, the whole thing should work just fine without the line removed in this commit. (However, there was no suitable spec to prove this, so some manual testing may be advisable. "Works fine for me". :-P)